The buoyancy control device, or BCD, is an expandable bladder that can be inflated with air from your tank to increase your buoyancy while diving. To decrease buoyancy, the BCD is deflated through special air-dump valves or hoses.

The BCD is a mandatory piece of equipment for all diving. It is used to provide positive buoyancy for resting, swimming or lending assistance to others Under water, the BCD allows you to maintain neutral buoyancy at any depth simply by adding or releasing air.

There are three basic styles of BCDs: 1)front mounted, 2)back-mounted and 3)jacket style. The jacket style BCD, which is worn like a a sleeveless coat, is the by far the most popular.

Regardless of what style you choose make sure to look for these five features:

1. It must hold enough air to give you and your equipment ample buoyancy at the surface.
2. It must have a large-diameter inflation/deflation hose, so air can be released easily.
3. It should have a low-pressure inflation system to make it possible to slowly fill your BCD with air directly from your tank. In addition the low-pressure inflation system should be easy to locate and operate.
4. It must have an over-pressure relief valve to prevent the BCD from rupturing if it is accidentally overfilled.
5. It should have a configuration and harness that makes wearing the BCD comfortable and keeps it from riding up around your neck when inflated.

When choosing a BCD, consider the five desirable features.

The BCD should fit as streamlined as possible against your body in order to prevent drag. Other desirable BCD features include a utility pocket, a whistle for surface communication, pockets for weights, hose retainers and utility rings for attaching accessory equipment.

This information was obtained from the Padi Open Water Diver Manual